The subject invention relates to fusion devices and, more particularly, to a novel first wall for such devices with high power density.
Fusion devices may be based upon the reaction of deuterium and tritium. Nuclei of deuterium and tritium, when brought together with sufficient energy, fuse to form a helium nucleus (an alpha particle), a neutron and 17.6 MeV of kinetic energy. In the conservation of energy, 14.1 MeV goes with the neutron (which has no charge), while the alpha particle (which has positive charge) carries 3.5 MeV.
Taken in transverse section, a central cell of a proposed fusion device includes a central plasma region in which fusion occurs, and a vacuum region encompassing the plasma region. Disposed next outwardly from the center of the cell is a first wall, and then a relatively thick blanket which might contain moderator material and tritium breeding material (lithium). A coolant such as helium, pressurized water or liquid metal is circulated through the blanket and first wall to maintain both at desired temperatures and to transport thermal energy from the device. Surrounding the blanket are magnetic coils which could be constituted by a toroidal, poloidal, helical and other coils or windings which may be either of superconductive or resistive material. The purposes of the magnetic fields are to produce and confine the plasma in the plasma region and away from the first wall. Examples of such fusion devices are disclosed in "WITAMIR-I, A University of Wisconsin Tandem Mirror Reactor Design", by Badger et al., UWFDM-400, University of Wisconsin, 1980; and in U.S. Pat. No. 4,302,284.
Because they have no charge, the high energy neutrons resulting from fusion within the plasma pass through the first wall with little loss of energy. They are slowed by multiple collisions with material in the blanket resulting in volumetric heating of the blanket. This heat can be relatively easily removed by the cooling system of the fusion device circulating coolant through the blanket.
On the other hand, the charged alpha particles drift across the plasma and are stopped by the first surface they hit. In the case of a typical fusion device this is the surface of the first wall facing the plasma. While the alpha particles carry only 20% of the total energy resulting from fusion, their stoppage results in surface heating, not volumetric heating. This heat must be transferred through the first wall structure from the plasma side to the coolant side, a task not easily accomplished when heat fluxes are high because of the high temperature gradients, and thermal stresses induced in the first wall structure by this heat flux.
Putting aside heat transfer considerations for a moment, the first wall of a fusion device has several requirements. It must be vacuum tight. It usually should have relatively high electrical resistance in the direction of the plasma path the first wall defines (toroidal in the case of the fusion device shown in U.S. Pat. No. 4,302,284). If the first wall has relatively low resistance, the magnetic field swing provided by one of the magnets would cause too large a current to flow in the plasma containment vessel defined by the first wall, thus unacceptably reducing the flow of current in the plasma. The first wall also must have erosion resistance because plasma particles strike and displace atoms on the first wall. Thus, a first wall is subject to somewhat conflicting requirements: high heat flux capability, relatively high electrical resistance and the always desirable low weight consideration all suggest a thin first wall structure; while erosion resistance and structural integrity suggest a thick structure, and the vacuum integrity requirement dictates a continuous structure.
It has been suggested that clad materials should be developed for the protection of a first wall. They could be armor plates in the form of tiles mounted on backing plates. These are generally intended for fusion reactors having lower heat flux than the present invention. A first wall of niobium, stainless steel, or another metal having a graphite or carbon curtain thereover has also been suggested. For further information regarding the structure and theoretical operation of such fusion device first walls, reference may be made to the following articles: "Coatings And Claddings For The Reduction Of Plasma Contamination And Surface Erosion In Fusion Reactors" by M. Kaminsky, published by Argonne National Laboratory, Argonne, Ill. 1980 (Conference No. 8008108-1) and "Heat Transfer In Inertial Confinement Fusion Reactor Systems" by J. Hovingh, published by Lawrence Livermore Laboratory, 1979 (Conference No. 790802-77). It is also known to provide a toroidal container for electrical discharge apparatus, with the container having staggered apertures for increasing the loop resistance of the liner. For further information regarding the structure and operation of such a liner, reference may be made to British Pat. No. 931,912.